1. Field of the Invention
The present invention relates to an industrial method for preventing the polymerization of unsaturated organosilicon compounds in the preparation and handling thereof
2. Description of the Related Art
Organosilicon compounds having unsaturated organic functional groups, e.g. vinyl, acryl or methacryl groups, are widely employed as bonding agents between inorganic and organic materials, e.g. in sizes for glass fibers, as crosslinkers in organic polymers or for the treatment of fillers.
Processes for preparing such compounds comprise, for example, the reaction catalyzed by metal compounds between silanes having SiH bonds and multiply unsaturated organic compounds. A further customary route is reaction of chloroalkylsilanes with alkali metal (meth)acrylates. All these processes proceed exothermically at elevated temperatures. There is therefore a risk of polymerization of the products via the unsaturated organic group during the reaction, as a result of which product is lost and reaction apparatuses used have to be subjected to costly cleaning.
In addition, the silanes bearing unsaturated organic groups are usually purified by distillation, which likewise entails a considerable polymerization risk due to the thermal stress necessary for the distillation. Finally, there is also a risk of polymerization during storage of these compounds.
Various methods of minimizing the risk of polymerization of organosilicon compounds having unsaturated organic groups are known. U.S. Pat. No. 4,276,426 describes, for example, the synthesis of 3-methacryloxypropylsilanes from allyl methacrylate and various silanes having SiH bonds with rapid pump circulation of the reactants in a loop reactor, as a result of which polymerization can be prevented.
Numerous methods of preventing the polymerization of organosilicon compounds bearing unsaturated organic groups involve the use of free-radical polymerization inhibitors: DE 11 83 503 describes stabilization by addition of 50-500 ppm of hydroxyphenyl compounds such as hydroquinone or hydroquinone monomethyl ether together with 0.5-10% by weight of an alcohol which is soluble in water and the silane. DE 22 38 295 describes the use of quinones together with the corresponding enols. U.S. Pat. No. 4,563,538 describes stabilization of the unsaturated organosilicon compounds by means of 2,6-di-tert-butylbenzoquinone, while a combination of 2,6-di-tert-butylhydroquinone and methanol is used in U.S. Pat. No. 4,722,807. Another way is described in U.S. Pat. No. 4,894,398: here, stabilization of the unsaturated organosilicon compound is effected by addition of a sufficient amount of a hydroxylamine. DE 38 32 621 C1 describes the combination of two different polymerization inhibitors, consisting of a compound from the class of N,N′-disubstituted p-phenylenediamines and a compound from the class of 2,6-di-tert-butyl-4-alkylphenols. U.S. Pat. No. 4,780,555 describes a further method of preventing the polymerization of unsaturated organosilicon compounds: here, the combination of phenothiazine together with a gas atmosphere containing at least 0.1% by volume of oxygen, which is brought into contact with the unsaturated organosilicon compound, effects stabilization. A disadvantage of this method is the need for a defined amount of oxygen to be present, which is technically complicated, especially during a distillation, and is also disadvantageous in terms of safety. A further combination of compounds, which is described as having a stabilizing effect in U.S. Pat. No. 5,145,979, is a mixture of a sterically hindered phenol, an aromatic amine and an alkylamine. Further compounds which can be used for stabilizing organosilicon compounds bearing unsaturated organic functional groups are, for example, specific 2,6-dialkyl-4-N,N-dialkylaminomethylphenols, either alone or in combination with other compounds having a stabilizing effect (EP 0 520 477 B1), tertiary amines (DE 44 30 729 A1), nonaromatic, stable free radicals such as 2,2,6,6-tetramethylpiperidinyl oxide (“TEMPO”, U.S. Pat. No. 5,616,753, U.S. Pat. No. 5,550,272), N,N′-disubstituted p-quinodiimines (EP 0 708 081 B1), dialkylamides of unsaturated organic acids (e.g. in EP 0 845 471 A2), zinc salts of 2-mercaptobenzothiazole or dimethyldithiocarbamate (e.g. in EP 0 845 465 A2).
All the methods described have the disadvantages that relatively large amounts of stabilizing compound have to be added, generally 50-2000 ppm by weight based on the weight of the silane, that these compounds are often quite expensive, and that the methods described are often, as in the case of contacting with an oxygen-containing gas mixture, problematic in terms of safety. Furthermore, most of the compounds described still involve the risk, despite a stabilizing effect on unsaturated organosilicon compounds, that the unsaturated organosilicon compound will polymerize and thereby be lost. Finally, a further disadvantage is the fact that most of the compounds described as polymerization inhibitors are solids which can only be metered using complicated working steps or apparatuses.
U.S. Pat. No. 6,441,228 B2 describes the use of molybdenum-containing steel alloys in the synthesis of methacrylic acid. WO 2005/40084 describes the use of copper-containing alloys for the preparation, purification, handling or storage of ethylenically unsaturated compounds. A disadvantage here is the use of unconventional alloys.